White color in video

I would like to have an object isolated on white, but if I export the file and upload it to the stock I always see light gray in the generated thumbnail, not white. I have RGB white 255/255/255 in the background but it seems it doesn't work well...

It might be an RGB versus YUV issue (and happening at istock's end of the process). If the clips themselves look good in the istock previews and it's just the thumbnails looking wrong, I'd suggest creating and uploading your own thumbnails.

The CCIR video standard defines the valid color range for video to be 16-235. Not 0-255 like regular RGB uses. The rest is used for under the hood stuff like color syncing, burst etc. Most likely Final Cut is converting your 255,255,255 to 235,235,235 like it should for video compliance, but the preview quicktime conversion at Istock does not convert it back again for proper viewing on PC monitors.